Such heating walls are generally made of refractory bricks or blocks designed to withstand the high operating temperatures of a coke oven, on the order of 1,000.degree. C. An expeditious heating of the coal charge in the adjoining coking chambers is, of course, very desirable for economic reasons. This requires a high rate of heat transfer between the gases traversing the ducts and the chambers separated from them by respective layers of such blocks. The latter usually are made of silica which is a refractory material of reasonable dimensional stability at the operating temperatures employed. Its thermal conductivity, however, is not particularly high; thus, as discussed for example in German Pat. No. 2,161,980, the thickness of silica blocks bounding the heating ducts on the sides facing the coking chambers should be as small as possible.
For structural reasons it is, of course, necessary to preserve a certain minimum wall thickness. Since, furthermore, the temperature of the heating gases cannot be increased at will, another possible solution would be the replacement of silica by a more highly heat-conductive refractory material. A major attempt made with so-called magnesite blocks has proved unsuccessful because of the impossibility to control the reversible dimensional changes of these blocks with varying temperatures. In fact, all known refractory materials other than silica undergo a significant reversible thermal expansion at temperatures above 1,000.degree. C., though in most instances not as large as with magnesite blocks. Silicon carbide (SiC), which has the highest heat conductivity among the usual refractories, has a thermal coefficient of expansion about half that of magnesite which still makes it very risky to use this material by itself in building blocks for a coke-oven wall.
German Pat. No. 143,332 describes a unitary refractory building block for a retort comprising a massive frame spanned by a thin-walled web. Such a block does have a lower thermal resistance than one of uniform thickness made of the same material, yet the thickness ratio between the frame and the web must be held within bounds in order to avoid cracks resulting from excessive differences in thermal expansion. Moreover, the size of such a unitary block is limited from a manufacturing viewpoint. Blocks of this type, even when provided with external tongues and grooves, are also difficult to use in a wall structure of a coke oven in which longitudinal and transverse blocks bounding the heating ducts are to be positively linked with one another by interlocking formations.